The present development relates to an improved dehydrogenation catalyst bed system for olefin production utilizing classical processing techniques. Specifically, the catalyst system comprises a chromia/alumina dehydrogenation catalyst that further includes physically mixing the catalyst with at least one other component that is catalytically inert with respect to dehydrogenation or side reactions such as cracking or coking but that generates heat after being exposed to reducing and/or to oxidizing reaction conditions.
Dehydrogenation of aliphatic hydrocarbons to produce their complementary olefins is a well-known process. In the typical Houdry CATOFIN® process, an aliphatic hydrocarbon, such as propane, is passed through a dehydrogenation catalyst bed where the hydrocarbon is dehydrogenated to its complementary olefin, such as propylene, the olefin is flushed from the bed, the catalyst is regenerated and reduced, and the cycle is repeated. (See, for example, U.S. Pat. No. 2,419,997 and incorporated herein by reference.)
The CATOFIN® dehydrogenation process is an adiabatic, cyclic process. Each cycle includes a catalyst reduction step, a dehydrogenation step, a step to purge the remaining hydrocarbon from the reactor, and finally a regeneration step with air. Following this, the cycle begins again with the reduction step.
The dehydrogenation reaction is highly endothermic. Therefore, during the dehydrogenation step the temperature at the top of the catalyst bed decreases by as much as 100° C. This decrease in temperature causes a decrease in paraffin conversion.
In order to reheat the catalyst bed and remove coke that has deposited on the catalyst during the dehydrogenation step, the reactor is purged of hydrocarbon and then undergoes a regeneration step with air heated to temperatures of up to 700° C. Heat is provided to the bed by the hot air that passes through the bed and also by the combustion of the coke deposits on the catalyst. Reduction of the catalyst, with a reducing gas such as hydrogen, prior to dehydrogenation step also provides some additional heat.
During regeneration, the hot air flows from the top of the catalyst bed to the bottom, and the regeneration cycle is relatively short, so there is a tendency for the top of the bed to be hotter than the bottom of the bed. The lower temperature in the bottom of the bed does not allow full utilization of the catalyst and thus the yield is lower that what would be otherwise expected. Also, the coke distribution in the catalyst bed, which is not easily controlled, affects the amount of heat added at each location and the resulting catalyst bed temperature profile. These factors make control of the temperature profile in the bed difficult.
In the conventional HOUDRY CATOFIN® process, the reactor contains a physical mixture of a chromia/alumina catalyst and an “inert”. The volume ratio between the “inert” material and the catalyst depends on a number of factors including the type of hydrocarbon feed being used in the dehydrogenation process. For example, for a propane feed the “inert” material equal to about 50% of the total catalyst volume, whereas for an isobutane feed the volume of the “inert” can be as low as about 30% of the total catalyst bed volume.
The “inert” is typically a granular, alpha-alumina material of similar particle size to the catalyst that is catalytically inactive with respect to dehydrogenation or side reactions such as cracking or coking, but that has a high density and high heat capacity, so it can be used to store additional heat in the bed. The additional heat is then used during the dehydrogenation step. However, the “inert” is not capable of producing heat during any stage of the process.
Since dehydrogenation is a highly endothermic reaction, a constant challenge related to the Houdry process, and similar adiabatic non-oxidative dehydrogenation processes, has been to identify a commercially feasible means for improving the heat addition to the unit without using a catalytically active material that produces large quantities of unwanted side products. Thus, it would be advantageous to identify a catalyst additive that has a heat capacity and density comparable to the currently used alumina “inert”, and that does not participate to any great extent in the dehydrogenation reaction or side reactions such as cracking or coking, and that can be physically mixed with the catalyst before loading, but that generates heat as needed during the operation.